Within each trunion of such a mill that is known, there is disposed a hollow cylinder carrying a resilient Archimedes' screw on its outside surface for the purpose of feeding raw coal, and onto which the raw coal falls from the pipework so as to be conveyed into the drum. "Main" air is injected into the drum via the inside of the cylinder so as to entrain the grains as milled inside the drum. The space between the trunion and the screw serves to evacuate the pulverized coal and it is connected to a separator. Additional air is injected into the pipework upstream from the separator.
By way of example, one such mill is described in patent application FR 2 721 711.
That type of mill suffers from the following technical problems.
It turns out that small quantities of additional air are incapable of providing effective pre-drying of the coal, with air flow rate being low when the mill is operating at full load.
Raw coal dropping onto the blades of the resilient screw gives rise rapidly to major wear thereof. This wear is accelerated by the pulverized coal as mixed with air passing through said space between the trunion and the screw.
Thus, said space serves both to feed raw coal into the drum and also to extract pulverized coal for feeding to the separator. These two flows of matter in opposite directions generate turbulence and a flow that is disturbed, thereby giving rise to high headlosses and to poor distribution of air and of pulverized coal within the separator.